1. Field of Invention
The invention relates to a sheet feeder that feeds cut sheets and an image forming apparatus including the sheet feeder, and more particularly, to a device that separates and feeds sheets, one by one, by abutting leading edges of the sheets in a sheet feed direction against an inclined separating surface formed by separating plates.
2. Description of Related Art
Conventional image forming apparatuses, such as laser-beam printers, color ink-jet printers, facsimile machines and copying machines, include a sheet feeder that feeds cut sheets, one by one, to an image forming unit provided therein. As disclosed in U.S. Pat. No. 6,158,733 and Japanese Laid-Open Patent Publication No. 2001-278507, the sheet feeder includes an inclined sheet holder, which holds a stack of sheets thereon, a separating plate, which is disposed below the sheet holder and has an inclined separating surface, and a sheet feed roller that is attached to an end of a rotatable arm having a predetermined length so as to face the sheet holder. The inclined separating surface extends such that the inclined separating surface and an upper surface of the sheet holder form an obtuse angle therebetween. Thus, the sheets stacked on the sheet holder are held by the inclined separating surface of the separating plate with the leading edges of the sheets contacting the inclined separating surface.
As the sheet feed roller rotates while pressing a topmost sheet in the stack on the sheet holder, the leading edge of the topmost sheet, which is being downwardly fed, abuts against the inclined separating surface. At that time, with a reaction force from the inclined separating surface, the topmost sheet is conveyed in a state where the lower part of the sheet is bent in a direction orthogonal to a direction that the rest of the sheets extend (the topmost sheet is convexedly warped such that the surface of the topmost sheet comes away from the stack of the sheets). Before long, the trailing edge of the sheet comes away from the inclined separating surface and thus only the topmost sheet is separated from the stack of the sheets. After that, the separated sheet is fed to the image forming unit by a conveying roller provided in a sheet feed path, to form an image thereon. Then, the sheet having the image is ejected from the image forming apparatus.
Japanese Laid-Open Patent Publication No. 2000-168980 discloses a sheet feeder that includes a sheet holder disposed in a horizontal position, a fixed guide plate, and movable guide plates. The fixed guide plate is inclined such that an inclined angle (exterior angle), which is formed between an extension of a surface of the sheet holder and a surface of the fixed guide plate, becomes β (smaller than 90 degrees). The movable guide plates are provided at one end of the sheet holder and at regular intervals in a sheet width direction. The movable guide plates pivot about the one end of the sheet holder. The fixed guide plate is also provided at the one end of the sheet holder. The movable guide plates are maintained by urging springs so that an inclined angle, which is formed between the extension of the surface of the sheet holder and surfaces of the movable guide plates, becomes α (also smaller than 90 degrees but larger than the inclined angle β, in an initial state). A frictional coefficient of the inclined surface of the fixed guide plate is smaller than that of the inclined surfaces of the movable guide plates.
In the sheet feeder described above, when a sheet feeding operation is performed by the sheet feed roller in a state where sheets are fully loaded on the sheet holder, the leading edges of the sheets press the movable guide plates and thus the movable guide plates pivot. When the inclined angles of the inclined surfaces (inclined separating surfaces) of the movable guide plates become the same degree as the inclined angle β of the fixed guide plate, which is a gentler slope than the inclined angle α, the leading edges of the sheets abut against the inclined surface (inclined separating surface) of the fixed guide plate having the smaller coefficient and a distance between the leading edges of the sheets and the contacting portion of the sheet feed roller and the sheet becomes longer. Thus, the leading edge of the topmost sheet is bent and the topmost sheet is smoothly separated from the stack.
On the other hand, when a small number of sheets are loaded on the sheet holder, the leading edges of the sheets are located near the center of the rotation of the movable guide plates. In addition, the arm length of the rotation moment for rotating the movable guide plates (a distance between a support point of the movable guide plate to a point of action of the force) is short, so that the leading edges of the sheets cannot rotate the movable guide plates. However, the distance between the leading edges of the sheets and the contacting portion of the sheet feed roller and the topmost sheet is longer than the distance therebetween when the full of sheets are loaded on the sheet holder. Further, an angle between the rotatable arm of the sheet feed roller and the surface of the sheet is large. Therefore, a sheet conveying force becomes large. Thus, even when the movable guide plates do not pivot, the leading edge of the topmost sheet is bent along the inclined surfaces of the movable guide plates and thus the topmost sheet can be smoothly separated from the stack one by one. The inclined separating surface, which is formed by the movable guide plates and/or the fixed guide plate, includes a flat surface extending along the sheet width direction.
A sheet feed roller to be provided in the sheet feeder may not have a length across the entire width of the sheet. Recently, the length of the sheet feed roller is designed as short as possible in order to achieve miniaturization and low power consumption of the sheet feeder. Therefore, a small sheet feed roller is provided at substantially the middle of the sheet width direction to contact a portion of a middle area of the sheet with the sheet feed roller. Further, most sheet feeders feed cut sheets of different types and sizes by using the same mechanism. When large-sized sheets are fed, the sheet feed roller contacts only a part of the sheets in the sheet width direction to feed the sheets.
When soft or week sheets are fed by the sheet feeder that includes the above-described small sheet feed roller that contacts only a part of the sheets, the sheets are conveyed one by one while being warped such that the lower edges (leading edges in a sheet feed direction) of the middle areas of the sheets are abutting against a portion on an extension of the acting portion of the sheet feed roller, in the inclined separating surface. At that time, side areas of the sheets, which are a distance from the acting portion of the sheet feed roller in the sheet width direction, become free ends, which are free from the sheet feeding force by the rotation of the sheet feed roller. Therefore, during the sheet feeding operation by the sheet feed roller, both side areas of the sheets are kept substantially flat while the sheets are being conveyed.
When the above-described situation is viewed from a direction facing the sheets stacked on the sheet holder, the inclined separating surface includes a flat surface along the sheet width direction. With this structure, the lower edges (leading edges in the sheet feed direction) of the side areas of the sheets contact the inclined separating surface before the leading edges of the middle areas of the sheets contact the inclined separating surface. Because of this, the middle areas of the sheets will be further warped. Accordingly, the lower edges (leading edges in the sheet feed direction) of the middle areas of the sheets easily come off from the corresponding portion of the inclined separating surface.
On the other hand, however, the lower edges (leading edges in the sheet feed direction) of the sheets hardly contact a separating member, which is provided to the inclined separating surface, according to a width and/or a position where the separating member is provided. Thus, it becomes difficult to contact the leading edges of the sheets with the separating member, and the separating action by the separating member cannot be surely exerted on the sheets. This may cause a multi-feed problem (feeding two or more sheets at a time). Such a problem remarkably occurs when sheet to be fed are wide in width (the sheet is long in the sheet width direction) and when the sheet feed roller is short in length.